The present invention generally relates to the determination of the amount of liquid in a sample container. More particularly, the present invention provides a method for determining the interface levels of liquid components in a transparent sample container.
Many biological samples such as urine, blood serum, plasma, cerebrospinal fluid and the like, are stored in a closed sample container after collection or pretreatment. Popularly used sample containers are transparent glass or plastic tubes to facilitate visual or machine inspection of various characteristics of the enclosed sample liquid, including color, volume, uniformity, etc. When samples are to be used for biochemical analysts, it is desirable to know if the amount of available sample fluid contained in the tube is sufficient to provide adequate volume to perform a single or a number of analytical tests. It is most desirable to determine the total sample fluid volume available for testing prior to opening a closed sample container and prior to beginning a sample division or aliquot process to ensure that ample sample is available or to prioritize the tests to be completed if an insufficient amount of sample is available.
Known methods to determine the volume of assorted liquids within closed sample tubes include the use of a photo-electrical detector in conjunction with a radiation beam. Typically, the beam is passed through the tube and captured by the detector, the detected signal being processed to ascertain the level of liquid, usually by analyzing the signal in order to determine some special feature within the signal. The volume of sample available for testing may be determined, depending on the diameter of the tube.
U.S. Pat. No. 5,747,824 discloses an array of four infrared LED""s and an array of four phototransistor receivers with each LED and phototransistor mounted inside a radiation baffle. The LEDs are positioned in a substantially vertical array just outside one side wall of the cassette. The vertical line on which the LEDs are arranged is substantially parallel to the direction in which the fluid/air interface moves within the cassette. The LEDs are aimed upwardly at an angle of approximately 20 degree(s) from horizontal. A corresponding substantially vertical array of four phototransistor receivers is mounted outside the cassette opposite the LEDs such that each of the receivers is aimed at its corresponding LED.
U.S. Pat. No. 5,274,245 discloses a device for detecting a specific liquid level, which can be mounted externally on a transparent or translucent vessel wall, and is insensitive to ambient radiation. This device utilizes a single radiation detector and a pair of AC activated radiation sources. The radiation sources produce reflected radiation signals which, when balanced at the detector, cancel. When liquid is absent the radiation signals are balanced, and no signal is detected. When liquid is present the extra signal reflected by the meniscus causes the radiation signals to become unbalanced, and a signal is detected.
U.S. Pat. No. 5,073,720 discloses a liquid level measurement device is an electro-optical device which uses a radiation source (typically a radiation emitting diode or laser diode) and an optical detector to measure the level of a liquid in a container. The radiation beam is passed through the liquid and received by the optical detector. The detector output is processed to determine the liquid level or the liquid volume.
U.S. Pat. No. 4,733,095 discloses a method for detecting a liquid level is monitored at mutually opposing sides of a liquid containing bottle. For monitoring the liquid level at each side of the bottle, a diffused radiation beam is irradiated toward the liquid surface from a level below the liquid surface. The radiation reflected at the liquid surface and the radiation refracted at the liquid surface are detected by a photo-receiving camera which is focused at a standard liquid level. An average value of the monitored liquid levels at both sides is taken as a liquid level indicative value.
U.S. Pat. No. 3,908,441 discloses a device for detecting the level of a liquid in a transparent tube including a radiation source to be placed facing a region of the periphery of the tube and a photocell to be placed facing a second zone of the tube periphery to receive radiation totally reflected from the internal face of the tube which has been wetted by a liquid, and which contains air rather than liquid. The device may be embodied in an automatic viscometer especially for colored or opaque liquids.
U.S. Pat. No. 3,636,360 discloses a method for photoelectronically detecting a liquid level by projecting a radiation beam to a transparent tube communicating with a pressure liquid tank and detecting the redirected radiation beam, utilizing the difference of relative refractivity at the inner wall of the tube when liquid fills the tube and when not, and the apparatus. Means for receiving the radiation beam redirected from the tube is so disposed as to receive the radiation beam only when the tube is empty, or the liquid is lower than the detector.
Accordingly, from a study of the different approaches taken in the prior art to the problems encountered with detecting the level of a liquid in a tube, there is a need for a simplified method, especially a method employing readily available detecting means.
Many of disadvantages of using complex analysis schemes and/or sophisticated equipment within the prior art are overcome by using the apparatus and/or methods of this invention. This invention provides a method for determining the level of a liquid in a sample tube by monitoring the intensity pattern of an radiation beam that has been transmitted through or reflected from the sample tube before and after the tube is tilted from an original alignment relative to the interrogating radiation beam. The captured radiation beam intensity pattern changes in those signal areas that correspond to the location of a liquid interface within the tube. By observing the position of such changes, it is possible to easily determine the levels of liquids having different refractive indices within the tube. Such a simplified approach makes it possible to accurately determine, for example, the upper level of a liquid within a closed sample tube without resorting to image analysis or other such techniques.
Associated with this method is an apparatus for automatically determining the liquid level within a sample tube using readily available electro-optical equipment in conjunction with a device for rotating a sample tube to expose a transparent portion of the sample tube surface to an interrogating optical beam and then tilting the tube to cause the level of liquid within the tube to vary its vertical position.